Add the SECCOMP_MODE_DISABLED, SECCOMP_MODE_STRICT and
SECCOMP_MODE_FILTER constants to be used with the PR_SET_SECCOMP prctl.
See http://man7.org/linux/man-pages/man2/prctl.2.html

Change-Id: Iacd1ffaeced4ae95e780a09038d35b763778a3d3
Reviewed-on: https://go-review.googlesource.com/46691Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>